The invention generally concerns a roller mounted on bearings, and is specifically concerned with a ball and socket joint between the ceramic roller and the bearings for eliminating localized stresses that can cause the roller to chip.
Rollers designed to support loads such as a metal strip in a heat-treatment furnace in which the strip is to be annealed at a relatively high temperature are already known. Such rollers are comprised of a cylinder of ceramic refractory material. An end cap mounted rotatably on a bearing is fixed at each end of the ceramic cylinder. U.S. Pat. No. 4,399,598 describes a ceramic roller for transporting glass sheets through a heat treatment. The cylinder is mounted rotatably on bearings by means of end caps mounted with play at each end of the cylinder. One or more radially compressible split metal rings are placed in the space between the outside diameter of the cylinder and the inside diameter of the end cap. A roller of similar type is also disclosed in U.S. Pat. No. 4,404,011; it is comprised of a cylinder of refractory material at the ends to which metallic end caps are fixed. Play is provided between the end caps and the cylinder. Flat leaf springs are placed around the periphery of this play.
These known systems of the prior art make it possible to take up the differential expansion between the roller and the end caps when the roller is exposed to heat. When heated, the cylinder of refractory material expands practically not at all while, on the other hand, the metallic end caps expand greatly. The play between the cylinder and the end caps consequently varies substantially as a function of the temperature. This is why it is necessary to provide resilient devices in this space that prevent backlash from occurring between the end caps and roller over a broad range of temperature.
However, these prior an systems have the shortcoming of not assuring coaxially between the end caps and the cylinder under load. In effect, when the cylinder supports a load, the resilient devices, whether involving the split rings disclosed in U.S. Pat. No. 4,399,598 or the leaf springs of U.S. Pat. No. 4,404,011, are deformed such that the end caps are displaced angularly with respect to the axis of the cylinder, thus causing the cylinder of the roller to rotate eccentrically with respect to the bearings.
To remedy these disadvantages, a roller having an end cap with a conical joint was developed as shown in U.S. Pat. No. 5,096,061. It is comprised of a ceramic cylinder, e.g., a vitreous fused silica, at the end of which at least one end cap is mounted. The junction between the cylinder and the end cap is a conical connection. The cylinder can be solid or comprised of a shell traversed by a metal shaft. In this latter case the end caps are mounted on the metal shaft, at least one of the end caps being applied resiliently (e.g., by means of helicoidal springs) against one end of the shell.
A roller of this type remedies the disadvantages associated with the prior art and assures coaxially of the cylinder and the end caps under load. This coaxially is not affected when even an elevated load is applied on the roller. Nevertheless, a device of this type also presents a number of shortcomings. In theory, the contact between the end caps and the cylinder (solid or hollow) is made along a frustoconical area, i.e., the conical engagement area of the roller ends on the conical surfaces of the end caps. In practice, because the conical surfaces of the cylinder and the end caps cannot be perfectly complementary due to machining tolerances, the contact is made only along an elongated zone on all sides of a contact generatrix. This contact zone rotates around the circumference of the cylinder-end cap interface with the rotation of the roller. In other words, when the roller turns, the contact between the end caps and the ends of the cylinder is progressively made over the entire periphery of the circumference of the interface. The metal shaft is deformed under load. Consequently, the contact zone, already reduced to an elongated zone located along the generatrix due to machining tolerances, is transformed into a zone of point support. The contact pressure between the end caps and the roller then increases considerably until it exceeds that admissible limit for the ceramic forming the cylinder. This then lead to a chipping and/or fissuring of the ceramic.